The strong desire in reducing the consumption of energy for heating and cooling of buildings has lead to many different and specialized insulation materials and techniques, and the prior art holds a countless number of specialized products and techniques, e.g.:                US 2004/0088939 A1, which teaches a facing of a faced insulation layer having Z-folded, double-folded, or single-folded lateral tabs extending the length of the facing sheet along or spaced inwardly from lateral edges of the facing sheet. One of the segments of each lateral tab has an adhesive thereon that can be exposed and extended beyond one of the lateral edges of the insulation layer for bonding the faced insulation layer to a framing member.        DE 3136935 C1, which teaches an insulation web incorporating for heat and sound insulation of buildings, in particular building roofs and external walls. The subject matter of the document is that the mineral wool is always formed within the web or panel from successive, mutually overlapping layers and in which the mineral fibres are ordered essentially parallel to the layer surfaces. The document also discloses a web including a vapour barrier intended to be mechanically secured to building elements.        U.S. Pat. No. 6,579,586 B1, which teaches a fibrous insulation batt encapsulated within an envelope to form an encapsulated insulation batt assembly. The envelope has pressure sensitive adhesive on lateral flanges or surfaces of the envelope for securing the encapsulated insulation batt assembly to spaced-apart frame members of buildings. Release liners, on surfaces of the envelope or the lateral flanges, which overlay and are releasable secured to the pressure sensitive adhesive, are removed from the pressure sensitive adhesive immediately prior to bonding the encapsulated insulation batt assembly to the spaced-apart frame members. Preferably, the release liners for the pressure sensitive adhesive are contact areas on the surfaces of the envelope or the lateral flanges coated or otherwise treated with a release agent.        U.S. Pat. No. 2,913,104 A1, which teaches encased insulation blankets with outwardly extending flanges intended to be mechanically secured to building elements, e.g. via nails or staples.        U.S. Pat. No. 5,362,539 A, which teaches a mineral fibre insulation assembly wherein the assembly includes a longitudinally extending mineral fibre core having opposed major surfaces, opposed side surfaces and opposed end surfaces. A low friction polymer film is positioned adjacent the major surfaces and the side surfaces. At least one of the side surfaces is attached to the polymer film. A plurality of openings are provided in the polymer film adjacent at least one of the side surfaces. The insulation assembly is readily compressible and expandable at the job site. The low friction film provides easy installation, however the low friction polymer film is glued to the core of the assembly.        
The focus on saving use of energy for heating and cooling of buildings has lead to the use of increasing thickness of the insulation layer. When insulating roofs, insulation is often arranged between rafters where it is important with a close fitting to the rafters in order to obtain the best insulation performance.
The present invention is based on the acknowledgement of a problem when installing such thick insulation between rafters. The problem arises when this thick insulation is also compressible e.g. for reasons of providing the cheapest possible transport from factory to building site. When unpacked at the building site the insulation will expand to the thickness it must have when installed.
It has been found that, when installing this insulation between beams or rafters, air gaps are formed which are not directly visible for the installer. These air gaps are extending along the direction of the rafters.
These air gaps are formed on the side opposite the side from which the insulation is installed, and are therefore not easily detected, or not realised during installation, to some extend because the installation of the insulation is performed as a task based contract resulting in a high speed of the work. However, such air gaps will considerably reduce the performance of the insulation and will result in higher costs for heating or cooling the building.
It has now been found that the cause of these air gaps is that the thick insulation will still be easily compressible when being installed and therefore the friction between the insulation material and the surface of the rafters will make it difficult to push the insulation material all the way into the correct position along the surface of the beams or rafters without the insulation being deformed. This leads to the formation of air gaps extending along the direction of the rafters.
None of the above cited documents realizes this disadvantage, and the objective of the invention has therefore been to find a solution to this new acknowledged problem of avoiding these air gaps without reducing the thickness or the compressibility of the insulation and without increasing installation time.